This invention relates generally to image reproduction by electronic halftoning; and more particularly relates to such a system including a technique for producing halftone screens at variable angles with minimum memory requirements.
The process of reproducing a continuous toned image by a halftone representation is well known in the art. Generally, areas in the original continuous toned image are approximated by dots of differing sizes where the gradation in dot sizes of the reproduction is dependent on the gray scale level in the area of the original. For very low density values in the original, small dots are used and for darker areas, a larger dot size is provided. These areas of differing sizes of dots, when viewed from a distance, appear to represent a continuous tone image because of the integration by the human eye. Therefore, the dot area or halftone cells are necessarily small in order to create a continuous tone illusion.
Such halftoning processes typically utilize optical screening techniques to produce the halftone cells. A screen having opaque rulings separating transmissive halftone cells were normally added optically to an image in order to form a resulting halftoned image. The optical halftone screen adds or multiplies the image, cell by cell, by its transmittance function. The combined image is then thresholded onto an imaging member, such as a photographic film to produce the halftoned image having areas of differing dot size. The size and shape of the halftone cell forming the final image is dependent upon the screen used and differing optical effects that are apparent in the final image can be modified by choosing the screen carefully.
With the advent of video scanning and electronic representation of an image came the idea of electronic halftoning. If an image can be generated electronically as a video signal, then one should be able to generate an "electronic screen" or a signal representative of such to combine with the image signal. The combined signal, if thresholded against a reference similar to the photographic technique, would then be an electronic halftone image that could be outputted to a recording medium by a transducer. The electronic halftoning systems in the prior art have represented both analog techniques and digital procedures. However, the storage in combination of the screen functions in the previous systems have been generally inefficient and relatively expensive. With the resolution of a final image being dependent upon the number of pixels scanned, prohibitive amounts of time and memory have had to be used to provide a quality reproduced image. Furthermore, and particularly in the digital electronic halftoning area, prior techniques fail to teach a simplified method for generating an electronic halftone screen at non-orthogonal angles. With variable angle screens, it is possible to change the texture and final appearance of an image. Certain screen angles are more pleasing to the eye and combination screens at multiple angles can be used to create effects not presently available. Further, color reproductions usually require screens at various angles so that Moire patterns are not seen in the final image.
In one prior technique of electronic halftoning, a simulation of the photographic process is achieved by individually turning on or off a large number of sub-cells from which is generated the electronic halftone cell or dot. A separate sample of the original image to be reproduced, a "pixel", is utilized in making a decision as to whether to turn on or off each sub-dot element, in the whole dot. In this method, there is combined typically be addition, a halftone screen function unrelated to original image intensity information with the electronic signal corresponding to the image information. This combined signal is then compared with a fixed threshold to determine how many partial dots within the halftone cell to turn on. Typically, levels above threshold are made white in the reproduction and levels below threshold are made black, although this is arbitrary and reverse may be true. Hence, the continuous tone original image becomes a binary image suitable for printing, display or viewing. In a digital implementation, signals for the screen and picture functions are sampled. Typically, there are twenty to thirty-two samples within the area corresponding to one period of the two dimensional screen function. Halftone dots of various sizes represent the gray levels.
Improvements over the usual technique are described by Klensch, R. J., "Electronically Generated Halftone Pictures", RCA Review, September, 1970 and Bayer, B.E., "An Optimum Method For Two-Level Rendition of Continuous-Tone Pictures", IEEE International Conference On Communications, Vol. 1, 1973.
The utilization of a rotated screen in other than a photographic system has previously been limited usually to zero degrees, or forty-five degrees. Forty-five degree screens are invariably based on patterns similar to those disclosed by B. E. Bayer, "An Optimum Method For Two-Level Rendition Of Continuous-Tone Pictures", International Conference On Communications, Conference Record, page 26-11 (1973) or B. Lipel and M. Kurland, "The Effects of Dither on Luminance Quantization Of Pictures", IEEE Transactions On Communication Technology, 6, page 879 (1971). A notable example can be found in C. N. Judice, et al, "Using Ordered Dither To Displace Continuous Tone Pictures On An AC Plasma Panel", Proceeding Of The S.I.D., Vol. 15/4, Fourth Quarter, 1974.
Digital electronic halftoning employs, in general, horizontal and vertical screen angles. Examples of these are found in "XCRIBL -- A Hard Copy Scan Line Graphics System For Document Generation", R. Reddy et al., Information Processing Letters (Netherlands), Vol. 1, No. 6, page 246 (1972). Analog systems have typically been limited to zero degree and/or forty-five degree screens. Typical analog systems are disclosed in R. J. Klensch et al., "Electronically Generated Halftone Pictures", Proceedings TAGA, page 302, 1970 and in R. L. Hallows et al., "Electronic Halftone", IEEE Spectrum, page 64, (October, 1968).
In new areas of technology it is often times desirable to provide improvements in systems which provide increased efficiency, greater capability and more variety than is presently obtainable. The present invention is such an improvement in that storage requirements for electronic half screening is reduced drastically; a great variety of halftone screen angles are permitted; and greater resolution is provided through the use of partial-dots of sub-dots within each halftone dot or cell.